Mycoplasma is the common name for a diverse group of cell wall-less procaryotes. Certain mycoplasmas are etiologic agents of plant, animal and human diseases. In humans, Mycoplasma pneumoniae causes primary atypical pneumonia. Some forms of human arthritis and nonspecific urogenital tract infections may also be caused by mycoplasmas. Few details exist defining mycoplasma pathogenicity. This study attempts to characterize mycoplasmas at the molecular level as an essential first step in comprehending host-parasite interactions. Current studies involve characterizing the viruses of mycoplasmas. The carrier state of these viruses and their potential for lysogenic conversion are being investigated. Polyacrylamide gel electrophoresis is being used to compare membrane proteins from infected, uninfected and virus-carrier cells. Interactions between viral, host chromosomal and plasmid DNAs are being characterized by ultracentrifugal and electrophoretic techniques. Tissue culture models will be used to ascertain the ability of infected, uninfected and virus-carrier mycoplasmas to interact with macrophages and other components of the vertebrate immune surveillance system.